Overexpression of Peroxisome Proliferator-activated Receptor γ Coactivator-1α Down-regulates GLUT4 mRNA in Skeletal Muscles

Abstract

Exercise training increases mitochondria and GLUT4 in skeletal muscles. Recent studies indicate that an increased expression of the transcriptional coactivator peroxisome proliferator-activated receptor ␥ coactivator 1␣ (PGC-1␣) by exercise may promote mitochondrial biogenesis and fatty acid oxidation. To examine whether increased PGC-1␣ expression was also responsible for an increase of GLUT4 expression, transgenic mice that overexpress PGC-1␣ in skeletal muscles driven by a human ␣-skeletal actin promoter were made. PGC-1␣ was overexpresssed in skeletal muscles including type I and II fiber-rich muscles but not in the heart. With an increase of PGC-1␣ mRNA, type II fiber-rich muscles were redder, and genes of mitochondrial oxidative metabolism were up-regulated in skeletal muscles, whereas the expression of GLUT4 mRNA was unexpectedly downregulated. In parallel with a decrease of GLUT4 mRNA, an impairment of glycemic control after intraperitoneal insulin administration was observed. Thus, an increase of PGC-1␣ plays a role in increasing mitochondrial biogenesis and fatty acid oxidation but not in increasing GLUT4 mRNA in skeletal muscles.Endurance exercise training improves physical performance by an enhancement of skeletal muscle respiratory capacity (1) and decreases the incidence rate of diabetes mellitus (2). These effects might be related to increased numbers of mitochondria and the GLUT4 isoform of the glucose transporter in skeletal muscles, respectively. GLUT4, the insulin-responsive glucose transporter, is expressed in skeletal muscles, heart, and adipose tissues, and the number of GLUT4 on plasma membrane becomes a rate-limiting step for whole-body glucose clearance under physiological conditions (3-5). Thus, exercise training modifies the skeletal muscle to metabolize more fatty acids and glucose.As for mitochondrial biogenesis, two transcription factors, nuclear respiratory factor (NRF)-1 1 and NRF-2, were the key transcriptional activators of nuclear genes encoding mitochondrial enzymes (6, 7). NRFs up-regulate mitochondrial transcription factor A, which stimulates mitochondrial DNA transcription and replication (8). Recently, as a regulator of NRFs, peroxisome proliferator-activated receptor ␥ coactivator-1 (PGC-1␣) was found. PGC-1␣ is an inducible coactivator of nuclear receptors cloned from brown fat cell cDNA library because of its interaction with peroxisome proliferator-activated receptor ␥ (PPAR␥) (9). Overexpression of PGC-1␣ increased expression of mitochondrial enzymes in 3T3 adipocytes, stimulated mitochondrial biogenesis in C2C12 myocytes (9, 10), and induced the expression of nuclear and mitochondrial genes involved in multiple mitochondrial energy production pathways, including PPAR␣ target genes encoding the mitochondrial fatty acid ␤-oxidation in neonatal cardiac myocytes (11). In vivo cardiac-specific overexpression of PGC-1␣ in transgenic mice resulted in uncontrolled massive proliferation of mitochondria (11). Also, evidence that PGC-1␣ co-activated PPAR␣ (12) and NRF-1 (10) le...

Increased levels of peroxisome proliferator-activated receptor gamma, coactivator 1 alpha (PGC-1α) improve lipid utilisation, insulin signalling and glucose transport in skeletal muscle of lean and insulin-resistant obese Zucker rats

“…A greater transfection efficiency requires higher electroporation voltages, which induce muscle damage (C. R. Benton and A. Bonen, unpublished data) [20]. Compared with transgenic Pgc-1α animals, transfection of a single muscle with Pgc-1α offers several advantages: (1) it is possible to examine PGC-1α-induced metabolic regulation in a controlled manner, in vivo; (2) this can be done without disturbing whole-body fuel homeostasis; (3) the animal serves as its own control; and (4) the resulting phenotype is not unusual, as is the case in Pgc-1α transgenic mice [10,11,36].…”

“…However, experimental studies in vivo have been disappointing. Transgenic Pgc-1α (also known as Ppargc1a) overexpression yielded an insulin-resistant phenotype [10,11], calling into question the therapeutic potential of PGC-1α. Alternatively, a model of transgenic Pgc-1α overexpression may have obscured the positive metabolic effects of this co-activator.…”

“…In transgenic animals, Pgc-1α mRNA overexpression is very large (6-to 21-fold increase) [10][11][12], well beyond that which occurs physiologically [13]. Excessive PGC-1α production induces many undesirable consequences, including abnormal mitochondrial proliferation, disruption of myofibrillar architecture, displacement of nuclei [12], obesity, intramuscular lipid accumulation and insulin resistance [10,11].…”

“…Excessive PGC-1α production induces many undesirable consequences, including abnormal mitochondrial proliferation, disruption of myofibrillar architecture, displacement of nuclei [12], obesity, intramuscular lipid accumulation and insulin resistance [10,11]. We [3,13] have suggested that insulin resistance in PGC-1α transgenic mice [10,11] may stem from a large, PGC-1α-induced increase in the fatty acid transporter, fatty acid translocase (FAT), which has been associated with increases in intramuscular lipids and insulin resistance in humans and in animals [14,15]. Viewed in this light, PGC-1α-induced insulin resistance in transgenic mice is less of a paradox than has been suggested [11].…”

Key signalling factors and pathways in the molecular determination of skeletal muscle phenotype

“…PGC-1a stimulates mitochondrial function and number (biogenesis), and fatty acid oxidation in cardiac and skeletal muscles by the induction of mitochondrial and nuclear genes involved in energy production pathways, including PPARa and nuclear respiratory factor-1 (NRF-1) (Miura et al, 2003). Therefore, the phenotypic effect of PGC-1a activation, in partnership with nuclear hormone receptors, including PPARs, in skeletal muscle is increased oxidative capacity.…”